Antennas that adjust the phase and amplitude of specific antenna elements are called active array antennas. The settings for associated phase/amplitude control devices are called antenna weights or a weight set. Active array antennas are used to make the radiated energy preferentially transmit to or receive from certain directions. When active arrays are used in wireless communications, the weights are set so the antenna beam points in the direction of the intended receiver. During the transmission, the weights are fixed. The set of weights that result in the antenna beam pointing in a preferential direction are not unique, however; there are several sets of weights that can result in similar beams pointing in similar directions.
In general, it is not possible to build an antenna where all the energy is pointed in one beam; some small amount of energy is transmitted into unintended directions. These other unintended small beams are called sidelobes. The security of such communications is limited by the sidelobe emissions, as such sidelobes allow eavesdropping on such signals. This is undesirable. Attempts at addressing this problem have focused primarily on reducing the level of emission in the sidelobes but the information is not eliminated. Generally, beam shaping techniques are used to maximize the energy in the main beam lobe, specifying the small sidelobe emission. Power is then reduced so that the main beam is just adequate to accomplish the mission.